1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and relates to a technique for separating an element formation layer including a semiconductor element from a substrate which is used at the time of manufacture.
In the present invention, a semiconductor device to be an object of manufacture includes a semiconductor element which can function by using semiconductor characteristics and general devices which function by using a plurality of semiconductor elements.
As the semiconductor element, a MOS transistor, a transistor such as a thin film transistor, a diode, a MOS capacitor, and the like are given as examples. In addition, the semiconductor device includes an integrated circuit having a plurality of semiconductor elements, a device having a plurality of integrated circuits, or a device having an integrated circuit and another element. The integrated circuit includes a memory circuit such as a CPU, a ROM, or a RAM, for example.
The device having a plurality of integrated circuits and the device having an integrated circuit and another element include, for example, a liquid crystal module substrate, a liquid crystal module using this module substrate, a liquid crystal display device using this module substrate, an EL (electroluminescence) module substrate, an EL module using this module substrate, an EL display device using this module substrate, an electronic device in which the liquid crystal module or the EL module is used as display means, an IC chip which is provided with an antenna and capable of wireless communication, an electronic tag provided with such an IC chip, an IC card, and the like.
2. Description of the Related Art
A technique has been developed, in which after an integrated circuit is formed of a semiconductor element such as a thin film transistor (TFT) over a base material such as a glass substrate or a quartz substrate, the base material used in manufacturing the integrated circuit is transferred to a plastic film base material. A step of separating the integrated circuit from a substrate used in manufacture is necessary to transfer the integrated circuit to another base material. Therefore, a technique has been developed, in which the integrated circuit is peeled from the substrate.
For example, the following peeling technique using laser ablation is described in Reference 1 (Japanese Published Patent Application No. H10-125931). A separation layer formed of amorphous silicon or the like is provided over a substrate, a layer to be peeled which is formed of a thin film element is provided over the separation layer, and the layer to be peeled is attached to a transfer body with an adhesive layer. The separation layer is ablated by laser beam irradiation, so that peeling is generated in the separation layer.
In addition, a technique in which peeling is performed with physical force such as human hands is described in Reference 2 (Japanese Published Patent Application No. 2003-174153). In Reference 2, a metal layer is formed between a substrate and an oxide layer, and a layer to be peeled and the substrate are separated from each other by using weak bonding at an interface between the oxide layer and the metal layer and generating peeling at the interface between the oxide layer and the metal layer.
It is known that, when peeling is generated, electric charge is generated on surfaces of two separated layers to be easily charged. This phenomenon is called peeling electrification. Since surfaces of two layers are close to each other at the moment when peeling is generated, capacitance is generated between these surfaces. When peeling proceeds, the capacitance is reduced with increase in a distance between two layers. However, the amount of electric charge generated by peeling electrification is not changed; therefore, a potential of the surface of the layer increases in inverse proportion to the capacitance. When the potential of the surface of the peeled layer increases, electric charge which is charged on the surface of the layer discharges toward inside of the layer in some cases.
Accordingly, when an object to be peeled is an integrated circuit, a semiconductor film, an insulating film, a conductive film, or the like is destroyed by being melted due to heat generated by discharge. As a result of this, a semiconductor element does not function in some cases. Even when the semiconductor element can operate without receiving damage which can be seen by appearance, a semiconductor or an insulator deteriorates due to high potential application and the semiconductor element does not show expected characteristics in some cases. Therefore, when discharge due to static electricity is generated, there is a possibility that the semiconductor element can be destroyed, or the integrated circuit itself using the semiconductor element cannot be operated normally due to characteristic deterioration.
Destruction of a semiconductor element or the like due to electrostatic discharge (hereinafter referred to as “ESD”) is called an electrostatic breakdown. The electrostatic breakdown is one of causes which greatly reduce yield. As a conventional method for avoiding the electrostatic breakdown, there are a method in which discharge due to static electricity is not generated and a method in which damage caused by discharge to the semiconductor element is suppressed even when discharge is generated due to static electricity. As the former method, a method for eliminating generated static electricity by providing an ionizer in semiconductor manufacturing equipment is known. A typical example of the latter method is a method for manufacturing a protection circuit with a semiconductor element, and a high potential generated by discharge can be prevented from being applied to the semiconductor element because of the protection circuit
Even when static electricity is generated, the electrostatic breakdown is not generated as long as discharge does not occur. Discharge is easy to be generated when a potential difference between two objects is large. Accordingly, the ionizer is a device for supplying a positive ion and a negative ion to the air which serves as a path of discharge and for not generating a large potential difference which brings discharge between the objects. However, since discharge due to peeling electrification is an instantaneous event in which two layers are separated, elimination of electric charge by the ionizer is not in time in some cases.
In addition, in the case of providing a protection circuit, when electric charge of discharge passes through the protection circuit, the protection circuit functions; therefore, destruction of a semiconductor element can be avoided. However, in peeling electrification, since the surfaces of two layers to be separated are charged, the pass of discharge does not always pass through the protection circuit. Accordingly, in peeling electrification, the electrostatic breakdown is not sufficiently prevented by the protection circuit.
For example, a method for preventing discharge due to peeling electrification is described in Reference 3 (Japanese Published Patent Application No. 2005-79395, referred to Scope of Claims, Lines 42 to 48 in Page 9). A conductive film is formed over a substrate, and a stack including a semiconductor element or the like is formed thereover. By generating peeling at an interface between the substrate and the conductive film and diffusing electric charge generated in peeling in the conductive film, destruction and characteristic deterioration of the semiconductor element due to electric charge is avoided.
However, by a peeling method of Reference 3, the conductive film remains in a lower portion of the stack. Depending on an intended purpose of the stack, the conductive film becomes an obstacle and an expected intended purpose cannot be carried out because of the conductive film in some cases. In such a case, it is necessary to remove the conductive film in the peeling method described in Reference 3.